Sorry to hear about your hen.... If you have the heart to do it you may want to have a necropsy done by someone, or even yourself if you can.... may not be anything specific you can diagnose from external exam but internal may show a different story. If she was normal until she went back to laying it may be that she was laying internally and got infected from it, or had some other type of reproductive tract abnormality.My Buff Orpington died this morning at the age of around 22 months. She went broody in late March, and I acquired some fertile eggs for her to sit on. Many pecks later, she hatched 3 of 5 eggs. She mothered them until I thought they were large enough to fend for themselves. She laid exactly one egg after hatching the chicks. I moved her back in with her cousins, but she lost color in her comb and wattles and her tail was down, not up. I would guess she has been in this lethargic state for a couple of months. Last week, I moved her into one of the unused nest boxes. As of yesterday, she was still pecking and drinking, but not moving out. Today she is dead. If she were a human, she would be diagnosed with "failure to thrive." Otherwise, I have no clue.
No matter what the cause though... never easy to loose a bird like that, I'm sorry you lost her....